1. Field of the Invention
The present invention relates to an optical film and a method for manufacturing the same.
2. Background Art
In recent years, liquid crystal displays are widely used in uses such as liquid crystal panels and the like, e.g., liquid crystal televisions, personal computers, portable telephones and digital cameras. In general, a liquid crystal display has a liquid crystal member having provided polarizing plates on both sides of a liquid crystal cell, and display is performed by controlling the light from the backlight member at the liquid crystal member. Here, the polarizing plate comprises a polarizer and the protective films on both sides of the polarizer. Ordinary polarizer can be obtained by dyeing a stretched polyvinyl alcohol (PVA) film with iodine or a dichroic dye, and a cellulose ester film or the like is used as the protective film. Cellulose ester films are high in transparency and optical isotropy, so that they are excellent as optical films.
Large-sizing and thinning of liquid crystal displays are progressing in recent years and uses are also diversifying, so that requirement for durability has been made rigorous.
For example, with the progress of thinning of liquid crystal displays, there arises a problem such that light unevenness, which seems like a circle or an ellipsoid occurs on the screen, and the improvement thereof is required. As a cause of generation of such light unevenness, it is indicated that the contact of a backlight member and a polarizing plate on the backlight side of a liquid crystal panel member is liable to occur due to thinning of a liquid crystal display. It is thought that when a liquid crystal display is used for a long period of term in the state of a backlight member and a polarizing plate on the backlight side being in contact with each other or a liquid crystal display is used under high temperature high humidity environment, water content is liable to gather at the contact area and the water content penetrates through to the polarizer, as a result the performance of the polarizing plate deteriorates to thereby cause light unevenness. As one measure to prevent such light unevenness, improvement of the moisture resisting property of the protective film of a polarizing plate is exemplified.
For the purpose of improving the moisture resisting property of the protective film of a polarizing plate, optical films in which a great amount of an acrylic resin such as polymethyl methacrylate (PMMA) is added to a cellulose ester resin are disclosed in WO 09/047,924 and WO 09/081,607. However, it has been found that liquid crystal displays having sufficient contrast ratio cannot be obtained with the optical films disclosed in Patent WO 09/047,924 and WO 09/081,607.
Further, in the manufacture of an optical film, retardation control of a film and stretching for widening the width are generally carried out. However, it has been found that a film itself is softened by the addition of a large amount of an acrylic resin such as PMMA and a problem arises such that the film ruptures by stretching in high magnification.